Wireless Channels

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Wireless Channels

• Y. Richard Yang
• 01/15/2009

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Outline

• Recap
• Characteristic of wireless channels

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Recap Wireless and Mobile Computing

• Driven by technology and infrastructure
• wireless communication technology
• global infrastructure
• device miniaturization and capabilities
• software development platforms
• Challenges
• wireless channel unreliable, open access
• mobility
• portability
• changing environment
• heterogeneity

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Recap Overview of Wireless Transmissions
receiver
bit stream
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Fourier Transform Every Signal Can be Decomposed
as a Collection of Harmonics
Time domain
Frequency domain
1
1
0
0
t
t
ideal periodical digital signal
decomposition

• Two representations
• time domain frequency domain
• Knowing one can recover the other

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Examples
Try spectrum1.m and spectrum2.m
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Recap Modulation

• Objective
• encode digital data into analog signals at the
  right frequency range

• Basic schemes
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

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Modulation

• Modulation of digital signals known as Shift
  Keying
• Amplitude Shift Keying (ASK)
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)

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Example

• Suppose fc 1 GHz(fc1 1 GHz, fc0 900
  GHzfor FSK)
• Bit rate is 1 Mbps
• Encode one bit at a time
• Bit seq 1 0 0 1 0
• Q How does the wave look like for each scheme?

t
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Phase Shift Keying BPSK

• BPSK (Binary Phase Shift Keying)
• bit value 0 sine wave
• bit value 1 inverted sine wave
• very simple PSK
• Properties
• robust, used e.g. in satellite systems

Q What is the spectrum usage of BPSK?
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Spectral Density of BPSK
Spectral Density bit rate-------------------w
idth of spectrum used
b
fc freq. of carrier
Rb Bb 1/Tb
b
fc
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Phase Shift Keying QPSK

• QPSK (Quadrature Phase Shift Keying)
• 2 bits coded as one symbol
• symbol determines shift of sine wave
• often also transmission of relative, not absolute
  phase shift DQPSK - Differential QPSK

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Phase Shift Keying Comparison
fc carrier freq. Rb freq. of data 10dB 10
20dB 100
BPSK
A
QPSK
t
01
11
10
00
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Quadrature Amplitude Modulation

• Quadrature Amplitude Modulation (QAM) combines
  amplitude and phase modulation
• it is possible to code n bits using one symbol
• 2n discrete levels

• Example 16-QAM (4 bits 1 symbol)
• Symbols 0011 and 0001 have the same phase f, but
  different amplitude a. 0000 and 1000 have same
  amplitude but different phase

Q why would any one use BPSK, but the highest
QAM?
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Antennas and Signal Propagation
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Antennas Isotropic Radiator

• Isotropic radiator a single point
• equal radiation in all directions (three
  dimensional)
• only a theoretical reference antenna
• Radiation pattern measurement of radiation
  around an antenna

z
z
y
ideal isotropic radiator
y
x
x
Q how does power level decrease as a function of
d, the distance from the transmitter to the
receiver?
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Free-Space Isotropic Signal Propagation

• In free space, receiving power proportional to
  1/d² (d distance between transmitter and
  receiver)
• Suppose transmitted signal is x,received signal
  y h x, where h is proportional to 1/d²

• Pr received power
• Pt transmitted power
• Gr, Gt receiver and transmitter antenna gain
• ? (c/f) wave length

Sometime we write path loss in log scale Lp
10 log(Pt) 10log(Pr)
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Free Space Signal Propagation
at distance d
?
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Real Antennas

• Real antennas are not isotropic radiators
• Some simple antennas quarter wave ?/4 on car
  roofs or half wave dipole ?/2 ? size of antenna
  proportional to wavelength for better
  transmission/receiving

Q Assume frequency 1 Ghz, ? ?
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Dipole Radiation Pattern of a Dipole
http//www.tpub.com/content/neets/14182/index.htm
http//en.wikipedia.org/wiki/Dipole_antenna
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Why Not Digital Signal (revisited)

• Not good for spectrum usage/sharing
• The wavelength can be extremely large to build
  portal devices
• e.g., T 1 us -gt f1/T 1MHz -gt wavelength
  3x108/106 300m

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Figure for Thought Real Measurements
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Signal Propagation

• Receiving power additionally influenced by
• shadowing (e.g. through a wall or a door)
• refraction depending on the density of a medium
• reflection at large obstacles
• scattering at small obstacles
• diffraction at edges

diffraction
refraction
scattering
shadow fading
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Signal Propagation Scenarios

• Details of signal propagation are very
  complicated
• We want to understand the key characteristics
  that are important to our objective

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Shadowing

• Signal strength loss after passing through
  obstacles
• Some sample numbers

i.e. reduces to ¼ of signal10 log(1/4) -6.02
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Multipath

• Signal can take many different paths between
  sender and receiver due to reflection,
  scattering, diffraction

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Multipath Can Reduce Signal Strength

• Example reflection from the ground received
  power decreases proportional to 1/d4 instead of
  1/d² due to the destructive interference between
  the direct signal and the signal reflected from
  the ground

For detail, see page 9 http//www.eecs.berkeley.
edu/dtse/Chapters_PDF/Fundamentals_Wireless_Commu
nication_chapter2.pdf
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Multipath Fading

• Due to constructive and destructive interference
  of multiple transmitted waves, signal strength
  may vary widely as a function of receiver position

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Multipath Fading A Simple Two-path Example
d2
d1
receiver
- Wavelength is about 0.3 m for 1 GHz cellular
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Multipath Fading with Mobility A Simple Two-path
Example
r(t) r0 v t, assume transmitter
sends out signal cos(2? fc t)
r0
More detail see page 16 Eqn. (2.13) http//www.ee
cs.berkeley.edu/dtse/Chapters_PDF/Fundamentals_Wi
reless_Communication_chapter2.pdf
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Received Waveform
10 ms
v 65 miles/h, fc 1 GHz
fc v/c 109 30 / 3x108 100 Hz
Why is fast multipath fading bad?
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Small-Scale Fading
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Multipath Can Spread Delay
signal at sender
LOS pulse
Time dispersion signal is dispersed over time

multipath pulses
signal at receiver
LOS Line Of Sight
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Delay Spread
RMS root-mean-square
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Multipath Can Cause ISI

• dispersed signal can cause interference between
  neighbor symbols, Inter Symbol Interference
  (ISI)
• Assume 300 meters delay spread, the arrival time
  difference is 300/3x108 1 ms
• if symbol rate gt 1 Ms/sec, we will have serious
  ISI
• In practice, fractional ISI can already
  substantially increase loss rate

signal at sender
LOS pulse
multipath pulses
signal at receiver
LOS Line Of Sight
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Summary Wireless Channels

• Channel characteristics change over location,
  time, and frequency

Received
Signal
Large-scale fading
power
Power
(dB)
path loss
log (distance)
time
small-scale fading
frequency